1. Field of the Invention
This invention relates to the art of submersible electric motor driven inducer equipped centrifugal pump units especially useful as cargo pumps for tanker ships and storage tanks and capable of pumping cargo such as cryogenic fluids or fluids at their boiling points. Particularly, the invention deals with the mounting of the inducer and centrifugal pump impellers on a motor shaft at opposite ends of the electric motor to eliminate inducer developed prerotation of the fluid in the impeller inlet, to make possible the use of a variety of inducer sizes without concern for the heretofore required relationship between an inducer outer diameter and impeller inner diameter, to simplify multi-staging of the pump and to balance inducer thrust with impeller thrust in a multistage pump.
2. Prior Art
My prior U.S. Pat. Nos. 3,304,877 issued Feb. 21, 1967; 3,369,715 issued Feb. 20, 1968; and 3,764,236 issued Oct. 9, 1973 disclose and claim submersible electric motor driven cargo pumps for the pumping of cryogenic fluids or fluids at their boiling points wherein an inducer impeller is mounted on and driven by the electric motor driven pump shaft in the pump inlet immediately ahead of the pump impeller. In these pumps, relatively small diameter inducer impellers are mounted in a relatively small pump inlet to discharge into an inner diameter inlet of the adjacent pump impeller. The adjacent mounting of the inducer and pump impellers required a restricted relation between the inducer outer diameter and the impeller inner diameter, thus, limiting the inducer size and the inlet diameter of the pump. The inducer created considerable prerotation of the fluid in the impeller entrance making the sum of the inducer and impeller heads no more than that of the impeller alone. While the pumps of these patents provided a good single stage pump under most conditions, they were not easily converted to multiple stage pumps and were not suitable for severe operating combinations of head and capacity conditions encountered in some installations. Further, the lower pump shaft bearing adjacent the pump impeller was subjected to severe loads from the inducer.